Hand-writable polymer dispersed liquid crystal board set with high resistance layer of crosslinking polymer adjacent conductive layer

ABSTRACT

A liquid crystal display device of externally charging type, including a screen from the outside of which display and erasing operations can be repeatedly performed simply, and having an excellent memory retention performance of the recorded image and durability. Also, a hand-writable liquid crystal board set being excellent in operativity, performances, safety, including the above liquid crystal display device as a liquid crystal board. The liquid crystal display device includes a layered structure comprising, in the following order: a conductive layer; a transparent high-electric resistance layer having a volume resistivity of 10 13  Ω•cm or more in a 20° C. air at a relative humidity of 90%; a liquid crystal-dispersed polymer layer which has a volume resistivity of 10 13  Ω•cm or more in a 20° C. air at a relative humidity of 90%; and a transparent insulator layer. The hand-writable liquid crystal board set comprises: 
     (A) a board of the above liquid crystal display device, 
     (B) display means for displaying recording images onto the board surface, and erasing means for erasing the recorded images; and 
     (C) a power source for generating a surface potential between the surface of the liquid crystal board and the conductive layer by the display means; 
     wherein the display means employs a writing member electrically connected to one terminal of the power source which is connected at the other terminal to the conductive layer; and the erasing means employs an erasing member electrically connected to the conductive layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device ofexternally charging type and a hand-writable liquid crystal board setincluding the device. More specifically, the present invention relatesto a hand-writable liquid crystal board set comprising: a liquid crystaldisplay device of externally charging type in which the display orerasing of an image is carried out by varying the state of lightscattering in the liquid crystal using electro-optical effects, or theprovision or removal of static electric charges; display means todisplay an image on a board screen; erasing means for erasing therecorded image; and a power source for generating a surface potential onthe board screen using the display means.

2. Description of the Related Art

A lot of proposals have already been made relating liquid crystaldisplay devices using the electro-optical effects, such as currenteffect, electric field effect, or heat effect of liquid crystals. Ofthese, a liquid crystal display device disclosed in Japanese PatentApplication Laid-Open No.(sho)55-96922, comprises a pair of transparentconductive layers, and a liquid crystal-dispersed polymer layersandwiched therebetween in which micro-capsules hermetically holdingliquid crystal therein are scattered. Since this liquid crystal displaydevice effects the display by varying the alignment of liquid crystal byturning on and off the application of electric field between electrodes,it is possible to easily obtain a large-sized display screen with asimple structure. Nevertheless, this display device requires to usepolarizing plates thus exhibiting drawbacks that the display image isdark and the field of view is narrow.

U.S. Pat. No. 4,435,047 as well as Japanese-translated PCT PatentApplication Laid-Open No.(sho)63-501512, discloses a liquid crystaldisplay device which has a simple structure with no polarizing plate,thereby the above drawbacks are eliminated, and which facilitates theprovision of a large-sized display screen. For this purpose the liquidcrystal display device includes a pair of transparent electrodes, and aliquid crystal-dispersed polymer layer sandwiched between the electrodesin which liquid crystal is finely dispersed in the polymer matrix. Theliquid crystal display device having such a structure, however, cannotbe applied to a so called externally charging type liquid crystaldisplay, in which recording and erasing can be effected from the outsidesurface of the display screen.

As an improvement in this respect, Japanese Patent Application Laid-OpenNos.(sho)61-83521 and (hei)3-43715 disclose a liquid crystal displaydevice comprising a liquid crystal-dispersed polymer layer in whichliquid crystal is finely dispersed in the polymer matrix and aconductive layer being coated in layer thereon; and an image displaymethod in which a recording image is formed by providing static chargeson the display screen from the outside of the liquid crystal-dispersedpolymer layer (of the display screen) and thus changing the phase and/oralignment of the liquid crystal.

However, none of these liquid crystal display apparatus describedheretofore would allow the hand-writable or erasing of images on thedisplay screen from the outside of the liquid crystal display device.

A liquid crystal display apparatus disclosed in Japanese PatentApplication Laying-Open No.(sho)61-83521, is to perform display anderasing of the record image by providing or removing static charges in aform of corona charge flow from the outside of the display screen of theliquid crystal display device. The present inventors hereof haveinvestigated liquid crystal display apparatuses of this kind, howeverand have found the fact that the provision of electric charges to thedisplay screen could cause the cholesteric liquid crystal to change thephase thereof to form a recording image, but the removal of the staticcharges could not bring back the phase-changed cholesteric liquidcrystal to the initial state, or could not erase the recorded image. Inorder to erase the recorded image, it was found that the liquid crystalmust be heated above the phase transition temperature, or be furtherprovided with an electrically conductive sheet through which an a.c.electric field can be applied. It was also found that, for repetitionsof display, the remaining static charges must be removed to effect nextdisplay. In sum, it was found that complicated operations were requiredto display and erase recorded images repeatedly.

In a liquid crystal display device disclosed in Japanese PatentApplication Laid-Open No.(hei)3-43715, if electric charges are providedto the display screen from the outside thereof by any of variouscharging techniques, the liquid crystal is aligned and this changes thestate of light scattering in the liquid crystal display device todisplay a recorded image. This disclosure teaches that, in order toerase the recorded image thus obtained, it is necessary to provideopposite electric charges equivalent to the amount of the static chargespresent only to the area of the recorded image. In practice, however, itis very difficult to perform such an operation since the amount of theopposite charges may be slightly more or less than that of the staticcharges. This situation makes the erasing operation uneasy. Moreover,the apparatus of the disclosure disadvantageously exhibits shortness ofmemory retention of the recorded image in a 20° C. air at a relativehumidity of 90% or under environment of high humidity.

Some hand-writable liquid crystal board sets have been tried to bemanufactured by utilizing the above described liquid crystal displayapparatuses, but the reasons described above disturbs the achievement.In other words, the former apparatus needs complicated display anderasing processes to repeatedly display images and erase them, or isproblematic in its handling and performances. For the latter apparatus,erasing of the recorded image is unfeasible, and moreover, the apparatusis problematic in its durability of the liquid crystal board and in itsperformance of memory retention of the recorded images under a highhumidity environment. For these reasons, it has been impossible toobtain a hand-writable liquid crystal board having a good operativityand sufficient durability and performances thereof.

SUMMARY OF THE INVENTION

The present invention has been achieved to eliminate the problems in theabove discussed prior art, and it is therefore an object of the presentinvention to provide a liquid crystal display device of externallycharging type which readily provides a display screen for a large sizeddisplay apparatus with a simple structure, and is facilitated to effectrecording and erasing from the outside of a display screen as well ashas an excellent memory retention performance of the recorded images,and which has practical strength such as heat resistance, etc. when usedas a display board. Further object of the present invention is toprovide a hand-writable liquid crystal board set including the aboveliquid crystal display device as the liquid crystal board, and beingexcellent in operativity, performances and safety. Specifically, thepresent invention is to provide a hand-writable liquid crystal board setwhich can be used, for example, as a toy or in education field, andtherefore, which can be used with safety meeting the practical utilityand which would give no harmful effect even if, in recording or erasingby hand-writable a coating material of a conductive lead memberconnected to an power source to make an electrical contact is broken andthe conductive lead member is brought into direct contact with any partof human body.

The present inventors have intensively studied on the hand-writableliquid crystal board set in order to achieve the above objects. As aresult, they found that a liquid crystal device and a hand-writableliquid crystal board set having a constitution as follows can provideoperativity, performances satisfying the practical utility, and havecompleted the invention.

The liquid crystal display of externally charging type of the presentinvention can be characterized by the following first item to thirditem.

(1) The liquid crystal display device of externally charging type inaccordance with the present invention comprises: a conductive layer; atransparent high-electric resistance layer having a volume resistivityof 10¹³ Ω•cm or more in a 20° C. air at a relative humidity of 90%; aliquid crystal-dispersed polymer layer, in which liquid crystal isfinely dispersed in polymer matrix thereof and which has a volumeresistivity of 10¹³ Ω•cm or more in a 20° C. air at a relative humidityof 90%; and a transparent insulator layer, wherein all of the layers aresuccessively layered in the order.

(2) The liquid crystal display device of externally charging typeaccording to item 1, wherein said high-electric resistance layercomprises a polymer or a film having a cross-linking structure.

(3) The liquid crystal display device of externally charging typeaccording to item 1, wherein the poller matrix of said liquidcrystal-dispersed polymer layer is composed of a polymer having across-linking structure.

The hand-writable liquid crystal board set of the present invention canbe characterized by the following fourth item to twelfth item.

(4) The hand-writable liquid crystal board set in accordance with thepresent invention comprises:

(A) a liquid crystal board of externally charging type comprising: aconductive layer, a transparent high-electric resistance layer having avolume resistivity of 10¹³ Ω•cm or more in a 20° C. air at a relativehumidity of 90%, a liquid crystal-dispersed polymer layer in whichliquid crystal is finely dispersed in polymer matrix thereof and whichhas a volume resistivity of 10¹³ Ω•cm or more in a 20° C. air at arelative humidity of 90%; and a transparent insulator layer, wherein allof the layers are successively layered in the order,

(B) display means for displaying recording images onto the board surfaceof the liquid crystal board, and erasing means for erasing the recordedimages; and

(C) a power source for generating a surface potential between thesurface of the liquid crystal board and the conductive layer by thedisplay means;

wherein the display means employs a writing member electricallyconnected to one terminal of the power source which is connected at theother terminal thereof to the conductive layer; and the erasing meansemploys an erasing member electrically connected to the conductivelayer.

(5) The hand-writable liquid crystal board set according to item 4,wherein said high-electric resistance layer comprises a polymer or afilm having a cross-linking structure.

(6) The hand-writable liquid crystal board set according to item 4,wherein the polymer matrix of said liquid crystal-dispersed polymerlayer is composed of a polymer having a cross-linking structure.

(7) The hand-writable liquid crystal board set according to item 4,wherein a lead wire which electrically connects said power source withsaid writing member, said writing member itself, a lead wire whichelectrically connects said conductive layer with said erasing member,and said erasing member itself all have a respective volume resistivityof 10¹² Ω•cm or less in a 23° C. at a relative humidity of 50%.

(8) The hand-writable liquid crystal board set according to item 4,wherein an electric resistance of 200 KΩ to 100 MΩ is electricallyconnected at least between one terminal of said power source and saidconductive layer, between one terminal of said power source and saiderasing member and between said conductive layer and said erasingmember.

(9) The hand-writable liquid crystal board set according to item 4,wherein an electric circuit including said display means and saiderasing means is equipped with an interlock device for preventing theoccurrence of electric shocks.

(10) The hand-writable liquid crystal board set according to item 4,wherein an electric circuit including said display means and saiderasing means is equipped with an automatic power-off device.

(11) The hand-writable liquid crystal board set according to item 4,wherein the board surface of said board, or said writing member and saiderasing member have been subjected to a dirt-protection treatment.

(12) The hand-writable liquid crystal board set according to item 4,wherein said writing member and/or said erasing member comprises afilm-like or sheet-like substrate having a pattern of a conductivematerial formed thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptional view showing a writing/drawing state of aliquid crystal display device (liquid crystal board) of externallycharging type according to the present invention;

FIG. 2 is a conceptional view showing an image-recorded state of aliquid crystal display device (liquid crystal board) of externallycharging type according to the present invention;

FIG. 3 is a conceptional view showing an image erasing state of a liquidcrystal display device (liquid crystal board) of externally chargingtype according to the present invention;

FIG. 4 is a conceptional view showing a hand-writable liquid crystalboard set with cord of the present invention;

FIG. 5 is a conceptional view showing a hand-writable liquid crystalboard set of a cordless type of the present invention;

FIG. 6 is a practical perspective view showing the hand-writable liquidcrystal board set shown in FIG. 4; and

FIG. 7 is a practical perspective view showing the hand-writable liquidcrystal board set shown in FIG. 5 .

1. liquid crystal display device (liquid crystal board) of externallycharging type

2. substrate

3. conductive layer

4. transparent high-electric resistance layer

5. polymer matrix

6. liquid crystal droplet

7. liquid crystal-dispersed polymer layer

8. transparent insulator layer

9. electric power source (built-in automatic power-off device)

10. terminal

11. electric resistance

12. lead wire

13. electric resistance

14. terminal

15. lead wire

16. a-contact of a switch for writing

17. lead wire

18. b-contact of a switch for erasing

19. lead wire

20. writing member (pen type)

20'. writing member (stamp type)

21. lead wire

22. b-contact of a switch for writing

23. lead wire

24. a-contact of a switch for erasing

25. electric resistance

26. lead wire

27. erasing member

28. switch for writing

29. board screen

30. recorded image

31. switch for erasing

32. contact type surface electrode of a switch for writing

33. contact type surface electrode of a switch for erasing

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A liquid crystal display device of externally charging type, or liquidcrystal board for use in a hand-writable liquid crystal board set of thepresent invention is composed mainly by successively forming in layeredstructure a conductive layer, a transparent high-electric resistancelayer having a volume resistivity of 10¹³ Ω•cm or more in a 20° C. airat a relative humidity of 90%, a liquid crystal-dispersed polymer layerin which liquid crystal droplets are dispersed in polymer matrix andwhich has a volume resistivity of 10¹³ Ω•cm or more in a 20° C. air at arelative humidity of 90%, and a transparent insulator layer.

The conductive layer of the liquid crystal board may be eithertransparent or opaque as long as the surface resistance thereof is notmore than 10⁷ Ω/□. In practice, a glass or a plastic film such aspolyesters, polyimides, etc. is used as a substrate, and the surface ofthe substrate is coated for conduction with aluminum, titanium,chromium, tin, rhodium, gold, stainless steel, titanium nitride,nickel-chromium alloy, aluminum-chromium alloy or indium tin oxide.

The liquid crystal-dispersed polymer layer as a component of the liquidcrystal display device of externally charging type of the inventionincludes finely dispersed liquid crystal in polymer matrix. As apreferable feature of the layer, droplets of liquid crystal are finelyand individually dispersed in the polymer matrix, in order to improvethe volume resistivity of the liquid crystal-dispersed polymer layer.

The ratio of polymer/liquid crystal in the liquid crystal-dispersedpolymer layer falls within a range of 9/1 to 1/9, and is preferably from7/3 to 5/5. Since finely dispersed liquid crystal drops having extremelylarge or much small size lower the performance of white-cloudiness formaking recorded images clear, the size preferably ranges from about 0.1to 20 micrometers. The thus obtained liquid crystal-dispersed polymerlayer preferably has a thickness of 1 to 50 micrometers.

The volume resistivity of the liquid crystal-dispersed polymer layer ispreferably 10¹³ Ω•cm in a 20° C. air at a relative humidity of 90%. Forthe resistivity of less than 10¹³ Ω•cm, the static charges for formingrecorded images cannot be retained balanced between the upper and lowerinterfaces of the liquid crystal-dispersed polymer layer, and are easyto move, thus the contours of the recorded images become vapor withoutsharpness with the passage of time.

As a polymer used for the liquid crystal-dispersed polymer layer, therecan be mentioned vinyl resin such as polyvinyl acetal, acrylic resin andthe like; polyester, polysulfone, polyphenylene oxide, ionomer,polycarbonate, polyolefin and the like.

Examples include silicone polymer and polymers containing a functionalgroup such as a double bond nitrile-, mercapto-, hydroxy-, carboxy-,epoxy-, Cl-containing, F-containing, isocyanate-, methoxy-, amino-, orchlorosulfone- group, or the like.

More specifically, there can be mentioned vinyl resins such as polyvinylacetal, acrylic resins and the like, epoxy resin, polyester, polyamide,vinylidene chloride resin, silicone resin, chlorinated polyethylene,chlorosulfonated polyethylene, polyphenylene oxide, styrene-butadienerubber, ionomer, fluororesin, etc.

Polymers as follows having a cross-linking structure (which will bereferred to as cross-linking polymers hereinafter) are preferably used.

Cross-linking polymers can be obtained by mixing a polymer having one ofthe aforementioned functional groups with a cross-linking agent reactivewith the functional group so as to cause a reaction between them, or bymixing a polymer having one of the aforementioned functional groups witha reactive polymer to cause a reaction between them. Cross-linkingpolymers do not exhibit any compatibility with liquid crystals at anelevated temperature, so that finely dispersed liquid crystal structurecan be maintained stable. Therefore, the liquid crystal-dispersedpolymer layer becomes durable and no deterioration of performancesoccurs after the passage of time.

Examples of the cross-linking agents used for the above-mentionedreaction, include organic peroxide, amine compounds, aziridinecompounds, epoxy compounds, dicarboxylic acids or anhydride ofcarboxylic acid, formaldehyde, dialdehyde, diols, bis-phenol, silanolcompounds, metallic oxide, metallic halide, photo-cross-linking agent(photopolymerization initiator), etc. Examples of the reactive polymersinclude phenol resin, amino resins, polyisocyanate, polyol, epoxy resin,etc.

As preferable cross-linking polymers, there can be mentionedcross-linking polymers formulated from a reaction between di- orpoly-isocyanate and a polymer such as polyvinylacetal resin, epoxyresin, acrylic resin having a functional group such as hydroxy group,carboxyl group, etc., polyester resin, fluororesin, and the like.

A liquid crystal used in the invention comprises a nematic liquidcrystal having positive dielectric anisotropy. The liquid crystal isrequired to have a mesomorphic temperature range of -10° to 100° C. tomatch with the practical utility as a liquid crystal board, andpreferably has indices of birefringence Δn of 0.2 or more, in order toimprove the cloudiness of the liquid crystal which serves to make therecorded image sharp. The liquid crystal may be added with a dichroicdye as required.

Dispersion of the liquid crystal in the polymer matrices can be carriedout by the polymer-liquid crystal common solvent evaporating separationprocess (common solvent casting process), the polymerizingphase-separation process in which precursor mixture of liquid crystaland polymer is polymerized by light or heat, the melt coolingphase-separation process in which liquid crystal and polymer are heatedto melt and then cooled, or by making the liquid crystalmicro-capsulized and dispersing them in the polymer.

A transparent polymer having high electric resistance can be used as thetransparent insulator layer for the liquid crystal board of theinvention. The volume resistivity of the layer is preferably 10¹³ Ω•cmor more at a relative humidity of 90% in a 20° C. air. The transparentinsulator layer is prepared such that a film made of, for example,polyethylene phthalate, polyethylene naphthalate, polypropylene,polyethylene, polyvinylchloride, polysulfone, polyphenylene oxide,ionomer, polycarbonate, nylon or fluororesin, is layered with a bond oradhesive or such that a solution of a material having the aforementionedphysical properties is applied and dried. Use of a material having avolume resistivity of less than 10¹³ Ω•cm at a relative humidity of 90%in a 20° C. air for the transparent insulator layer gives rise to aproblem in its performance due to the same reasons as stated for theliquid crystal-dispersed polymer layer, and therefore the thus obtainedliquid crystal board can not meet the practical utility.

A transparent high-electric resistance layer of the liquid crystal boardof the invention is disposed between the conductive layer and the liquidcrystal-dispersed polymer layer. The volume resistivity is 10¹³ Ω•cm ormore at a relative humidity of 90% in a 20° C. air. This layer moreassuringly prevents the static charges retained on the upper and lowerinterfaces of the liquid crystal-dispersed polymer layer from moving andattenuating, so that it can improve the performances in sharpness ofcontours of the recorded image, memory retention, erasability, etc. Useof a material having a volume resistivity of 10¹³ Ω•cm or less at arelative humidity of 90% in a 20° C. air for the transparenthigh-electric resistance layer gives rise to a problem in itsperformances due to the same reasons as stated for the liquidcrystal-dispersed polymer layer, and therefore the thus obtained liquidcrystal board can not meet the practical utility.

The following cross-linking polymers may be used for the transparenthigh-electric resistance layer.

Cross-linking polymers can be obtained such that one of silicone polymerand polymer having a functional group such as a double bond, nitrile-,mercapto-, hydroxy-, carboxy-, epoxy-, Cl-containing, F-containing,isocyanate-, methoxy-, amino-, chlorosulfone-group or the like is mixedwith a cross-linking agent reactive with the functional group to cause areaction between them, or such that one of the polymers having theaforementioned functional group is mixed with a reactive polymer to forma cross-linking polymer.

Examples of the cross-linking agents used for the aforementionedreaction, include organic peroxide, amin compounds, aziridine compounds,epoxy compounds, dicarboxylic acids or anhydrides of carboxylic acid,formaldehyde, dialdehyde, diol, bis-phenol, silanol compounds, metallicoxide, metallic halide, photo-cross-linking agent (photopolymerizationinitiator), etc. Examples of the reactive polymers include phenol resin,amino resin, polyisocyanate, polyol, epoxy resin, etc. As preferablecross-linking polymers, there can be mentioned cross-linking polymersformulated from a reaction between di- or poly-isocyanate, and a polymersuch as polyvinylacetal, epoxy resin, acrylic resins having a functionalgroup such as hydroxy group, carboxyl group, etc., polyester resin,fluororesin, and the like.

The transparent high-electric resistance layer can employ a polymerfilm. The polymer film can be prepared from polyethylene terephthalate,polyethylene naphthalate, polypropylene, polyethylene,polyvinylchloride, polysulfone, polyphenyleneoxide, ionomer,polycarbonate, nylon, fluororesin, silicon dioxide, or etc.

Each of the aforesaid liquid crystal-dispersed polymer layer, thetransparent insulator layer and the transparent high-electric resistancelayer is provided in layer by the application process in which asolution of the polymer is applied and dried, or by the laminatingprocess in which a film of the polymer is adhered with a bond oradhesive.

In the liquid crystal board of the present invention, in order to obtainan image having clear contrast, a dichroic dye may be added to theliquid crystal of the liquid crystal-dispersed polymer layer to changethe color tone of the board screen. On the other hand, if, withtransparent conductive layer and substrate, paper, plastic film or othermaterial is disposed as a coloring layer on the outer side of thesubstrate, any color can be allotted to the board screen and the image,thus making it possible to form a clear contrast image.

The display means for recording images used in the hand-writable liquidcrystal board set of the invention employs a writing member electricallyconnected to one terminal of a power source which is connected at theother terminal thereof to the conductive layer of the liquid crystalboard for generating a surface potential between the surface of theliquid crystal board and the conductive layer. When the writing memberis brought into contact with the board screen of the liquid crystalboard, it is possible to form a recording image.

If the surface potential is generated on the board screen, the liquidcrystal present in the liquid crystal-dispersed polymer layer is alignedby the static electric field, so that the aligned portions becometransparent in a white-clouded board screen, thus forming an image withhigh contrast. When the surface potential is less than a threshold levelabove which the liquid crystal in the liquid crystal-dispersed polymeris to be aligned, the liquid crystal cannot be aligned enough to becompletely transparentized. As a result, the recorded image is poor incontrast.

Erasing means for erasing recorded images in the hand-writable liquidcrystal board set of the present invention, includes an erasing memberelectrically connected to the conductive layer of the liquid crystalboard. When the erasing member is brought into contact with a recordedimage on the board screen (or more exactly on the surface of thetransparent insulator layer) of the liquid crystal board, the recordedimage and the conductive layer become equipotential, whereby the staticcharges present in the recorded image area are easily removed to be ableto erase the recorded image.

In the hand-writable liquid crystal board set of the present invention,the display means extending from the terminal of the power source to thewriting member as well as the erasing member extending from theconductive layer to the erasing member may be composed of a conductivematerial having a resistivity of 10¹² Ω•cm or less at a relativehumidity of 50% in a 23° C. air, and preferably of a conductive materialhaving a resistivity of 10⁸ Ω•cm or less. When drawing or erasingoperation is effected with the display or erasing means made of aconductor of 10¹² Ω•cm or less, it is possible to smoothly providestatic charges onto the board screen to align the liquid crystal, or toeasily remove the static charges and restore the initial state of theliquid crystal from the aligned state. As a result, an image with highcontrast, and a smooth erasing performance can be easily achieved. Onthe other hand, if the volume resistivity of the conductor is more than10¹² Ω•cm, the contrast of the recorded image becomes low, the responsebecomes dull, and the erasability becomes insufficient so that desirederasability cannot be obtained unless a lot of erasing operations arerepeated. This tendency is unpreferable for the hand-writable liquidcrystal board set.

Primarily in the present invention, transparent images are to be formedon, for instance, a white-clouded background of the board screen. Animage of white-cloudiness, opposing the original display, however canalso be obtained in the transparent board screen. This can be effectedby, at first, providing static charges onto the board screen entirely bymeans of the writing member to align the liquid crystal and next,removing part of the static charges from the board screen by means ofthe erasing member.

In the hand-writable liquid crystal board set of the present invention,the writing member used for the display means and the erasing memberused for the erasing means are to be connected electrically to theterminal of the power source and the conductive layer, respectively.Here, the "electrically connection" must not be assumed as a connectionby the lead wire. For example, the human body is electricallyconductive, so that it is possible to use part of human body for drawingand erasing. That is, while contacting an appropriate part of thedisplay means by one hand, an operator can touch the board screenthrough the writing member held by the other hand to thereby make animage in the same manner as with the above display means. In a similarmanner, while contacting an appropriate part of the erasing means by onehand, the operator can touch the board screen through the erasing memberheld by the other hand to thereby erase the recorded image in the samemanner as with the above erasing means. In this case, it is alsopossible to draw or erase an image directly with hands or fingerswithout any writing or erasing member.

In the hand-writable liquid crystal board set of the present invention,the writing member and erasing member can be composed of a metal or analloy of copper, iron, aluminum, tin, palladium, gold, silver, brass,stainless steel, etc. It is also possible to use metallic oxides such asof tin, indium, silver, zinc, manganese, etc.; conductive polymers suchas polypyrrole, polythiophene, polyaniline, etc; and conductivematerials such as graphite, carbon fiber, etc.

These materials can be used in various forms depending upon thecharacteristic of the material. That is, some material of these may bedirectly formed into a wire (lead wire), a contact (terminal) or aconductive fabric. On the other hand, coating the surface of a plasticfilm, fabric, nonwoven fabric, rubber, sponge, glass, etc. with one ofthe materials above allows a conductive film, conductive fiber,conductive rubber, conductive sponge, conductive glass, etc.Alternatively, some of the materials are processed into grains orfibers, and thus formed grains or fibers can be kneaded with a plasticor a rubber to form a conductive plastic, conductive rubber, conductiveporous member, etc. These products can be used in combination inaccordance with the object, or may be further processed so as to be usedas a writing member or an erasing member.

By preparing a writing member and/or erasing member in a form of a stampusing a conductive elastic material, it is possible to easily displayany letter or any pattern by contacting the stamp onto the board screen.Alternatively, using a film-like or sheet-like substrate having aconductive pattern formed thereon by a transparent or opaque conductiveink or conductive resin, it is possible to easily display a complicatedshape or pattern such as maps and the like on the board screen at onetime. Moreover, preparing conductive fibers or an elastic conductivebody of a calligraphic brush type and using it makes it possible to makethe trace of the recorded image vary narrower or wider as if it werewritten with a calligraphic brush.

The power source in the board set of the present invention is preferablycomposed of a substantially direct current power source which provides apotential difference between the conductive layer and the writing memberso as to generate a surface potential on the board screen. Any powersource, for example, even a solar battery may be used as long as itsatisfies the above requirement.

In the hand-writable liquid crystal board set of the invention, there isa possibility or fear of receiving an electric shock if an operatorcomes in contact with a part of the erasing means while touching a partof the display means for generating the surface potential on the boardscreen of the liquid crystal board. So this respect is to be considered.To deal with this problem, a safety mechanism (or interlock) wasdeveloped and is provided to the hand-writable liquid crystal board setin order to prevent the electric shock from occurring even when the bodyof the operator comes into contact with both the parts of the displaymeans and the erasing means simultaneously. The interlock mechanism usedhere, or in this hand-writable liquid crystal board set, works as insuch a configuration in which the power source provides a surfacepotential onto the board screen through the display means which isconnected at one terminal thereof and the other terminal of the powersource is connected to the conductive layer whereas the erasing meansfor erasing the recorded image on the board screen is connected to thatterminal of the power source which is connected to the conductive layer.In such the structure, the interlock operates as a safety device whichfunctions electrically or mechanically such that at least one of theterminals of the display and erasing means is adapted to disconnect whenan operator's body comes in contact with both the display means and theerasing means simultaneously.

In addition to the interlocking arrangement, electric resistors of 200KΩ to 100 MΩ are each provided between the conductive layer-sidedterminal of the power source and the conductive layer, between theerasing member-sided terminal of the power source and the erasingmember, and between the conductive layer and the erasing member. Theseresistors afford an improved safety to the hand-writable liquid crystalboard set, whereby no electric shock can be felt if the displaying meansand/or the erasing means are brought into contact with the operator'sbody. The resistance used here is allowed to be within a range of from200 KΩ to 100 MΩ for a generated voltage of 50 to 1000 volts, and morepreferably is limited within a range of from 1 MΩ to 40 MΩ. The reasonlimiting the resistance as such is because that, if any member of thedisplay and the erasing means is brought into contact with the humanbody through a resistance less than 200 KΩ when a 1000 V power source isused, 5 mA or more current flows through the human body to give anelectric shock thereto. On the other hand, if a resistance over 100 MΩis employed, a current leak is easy to occur due to the relation to theother insulating resistances in the substrate etc., resulting in avoltage drop in the generated voltage, or deterioration of the responsefor the display and erasing operations occurs resulting in inability toperform smooth operations of display and erasing. Since thehand-writable liquid crystal board set is provided with both theinterlock arrangement and the electric shock protecting measure based onthe resistances, the board set is highly improved in safety and can beused by anyone without fear or anxiety.

The hand-writable liquid crystal board set thus highly improved insafety can be applied to a cordless type set. In the hand-writableliquid crystal board set of cordless type, a power source is connectedat one terminal thereof to a conductive layer while the other terminalof the power source is connected through a resistance to a surfaceelectrode which works as press-button of a switch for writing. Theconductive layer is also connected through a resistance to anothersurface electrode which works as a press-button of a switch for erasing.Here, the switches for writing and erasing constitute interlockingstructures. Connected to one of the terminals of the power source is aseries of a b-contact of the switch for writing and an a-contact of theswitch for erasing, whereas a series of an a-contact of the switch forwriting and a b-contact of the switch for erasing is connected to theother terminal of the power source. With this arrangement, touching thesurface electrode of the switch for writing with the left hand to pressit, an operator can draw a recording image by touching the board surfacewith the right hand itself or the writing member held in the right hand.On the other hand, touch the surface electrode of the switch for erasingwith the left hand to press it, the operator can erase a recorded imageby touching the board surface with the right hand itself or the erasingmember held in the right hand. In both cases, no lead wire is required,therefore even though display and erasing are repeated in many times,there is no fear that the wires are entangled or disconnected, thusallowing the hand-writable liquid crystal board set to be simple andhard to break down.

In the hand-writable liquid crystal board set of the invention, theremay be provided an automatic power-off device to the electric circuitfor the display means. Since this device will automatically cut theswitch of the power source after the passage of a predetermined time ifthe switch has been left on-state, it is possible to make the apparatuspower-saving type which does not waste electric power.

In the hand-writable liquid crystal board set of the invention, theboard screen and the writing and erasing members can be impregnated orcoated with an oil. Alternatively, a polymer film may be coated on theseitems. With these treatment, the board screen will be hard to dirty. Theoil to be used should have a volume resistivity of 10¹³ Ω•cm or more at50% RH in a 23° C. air, a boiling point of 200° C. or more, insolubilityin water and a low surface tension. Examples of the oil includesinsulation oil, silicone oil, fluorine oil, etc. As the polymer,fluororesin can be exampled. Repetitions of display and erasingoperations on the board screen would leave oil, sweat, cosmetic or anyother dirt from the hands applied on the board screen little by littlefor a long period of time. This would cause the applied static chargesto be easy to move and therefore attenuate, resulting in deteriorationof the contour sharpness of the recorded image and lowering the memoryretention and the erasability. The above treatment serves to eliminateor prevent these adverse effects.

In the hand-writable liquid crystal board set of the invention, displayand erasing of the recording image is carried out based on a principlethat presence and absence of static electric fields in the liquidcrystal- dispersed polymer layer of the liquid crystal board createtransparent and opaque states on the liquid crystal display device ofexternally charging type. More specifically, droplet liquid crystal inthe liquid crystal-dispersed polymer layer is finely dispersed in thepolymer matrix and molecules of the liquid crystal is aligned along thepolymer wall. Accordingly, when the static charge is not supplied, thedifference in index of refraction between the polymer and the liquidcrystal causes light-scattering and make the liquid crystal boardwhite-clouded or opaque. On the other hand, if an electric field isapplied to the liquid crystal-dispersed polymer layer through thetransparent insulator layer by the display means, the liquid crystal isoriented in a direction of the electric field. By this alignment, theindices of refraction in the polymer and the liquid crystal becomesubstantially equal to each other. As a result, light scattering isinhibited to create transparent areas, or a contrast is generatedbetween the board screen and the recorded image so as to put the screenin the displaying state (FIG. 1). In order to erase the image, theelectric field created in the liquid crystal-dispersed polymer layer isto be removed by the erasing means. By the operation, the liquid crystalis brought back to the initial state or the state in which no electriccharge is present, and light is scattered again to bring the boardscreen in the white-clouded state (FIG. 3).

In accordance with the hand-writable liquid crystal board set of thepresent invention having the above-described structure, it is possiblefor anyone to easily write and draw images and erase the recorded imagesby hands using the simple display and erasing means, without necessityof a complicated apparatus and. troublesome operations. In addition,despite that electric operations, namely provision and removal ofelectric charges are carried out in displaying and erasing, the boardset is excellent in safety without fear to cause an electric shock tothe human body.

In accordance with the prior art, a liquid crystal display device isformed such that a liquid crystal-dispersed polymer layer is laminateddirectly onto a conductive layer. In such a liquid crystal displaydevice, provision of static charges is conducted by various methods,such as for example, voltage application, corona discharge, contactelectrical charging, etc., thus generating electric fields in theportions in which electric charges is provided. Although the electricfields thus generated make the liquid crystal align to form an image,the static charges move and therefore attenuate with the passage oftime. In other words, the electric fields attenuate and disappear withtime, so that the alignment of the nematic liquid crystal is broughtback to the initial state, resulting in insufficiency of the memoryperformance as a display device. In this invention, however, ahigh-electric resistance layer is disposed between the conductive layerand the liquid crystal-dispersed polymer layer, so that the performanceof memory retention is markedly improved. This can be considered thatthe high-electric resistance layer prevents the static charge frommoving, the electric field for the recorded image being retained (FIG.2), thus providing an excellent performance for memory retention of therecorded image which could not achieved by the conventional structure.

In the liquid crystal display device of the invention, the staticcharges in the recorded image portion can be removed easily byelectrically connecting the conductive layer with the image recordedportion or the surface of the transparent insulator layer so that thetwo portions may be equi-potential. As a result, the electric fielddisappears bringing the alignment of the liquid crystal back to theinitial state. In short erasing of the recorded image can be conductedin a simple manner (FIG. 3). In the conventional liquid crystal displaydevice, the static charges staying in the liquid crystal-dispersedpolymer layer on the side of the conductive layer move and attenuatewith the passage of time, so that the balance of the static chargeamount between the upper and lower sides of the liquid crystal-dispersedpolymer layer is lost. This imbalance of the charges makes it difficultto remove or eliminate the electric field from the surface of thetransparent insulator layer, resulting in a poor erasability. In thepresent invention, however, the provision of the transparenthigh-electric resistance layer also improves the erasability. This canbe considered that the high-electric resistance layer inhibits thestatic charge from moving, and the balance of the static charge amountbetween the upper and lower sides of the liquid crystal-dispersedpolymer layer is kept, thus providing an excellent erasability of therecorded image which could not achieved by the conventional structure.

Generally, the transparent insulator layer is provided for the purposeof protecting the liquid crystal-dispersed polymer layer. That is, theprovision is to prevent the liquid crystal-dispersed polymer layer whichworks as a display screen from being easily damaged due to the frictionand abrasion by the other members when recording and erasing areeffected. But the provision of the transparent insulator layer alsoprevents the attenuation of the static charge from the liquidcrystal-dispersed polymer layer even at the time of high humidity,specifically at a relative humidity of 90% in a 20° C. air. This makesthe recorded image clear without blurring at its contour with thepassage of time, and improves the memory retention performance and theerasability of the recorded image. This can be considered that thetransparent insulator layer keeps the balance of the static chargeamount between the upper and lower sides of the liquid crystal-dispersedpolymer layer.

According to the present invention, in the hand-writable liquid crystalboard set a large-sized board screen can be provided and display anderasing of the image can be effected from the outside of the boardscreen due to the characteristic of the liquid crystal board ofexternally charging type used. In addition, the memory retentionperformance of the recorded image is excellent and liquid crystal boardhas sufficient durability for practical use. Moreover, the hand-writableliquid crystal board set is designed so even a child can operate theboard set safely. Therefore, the apparatus of the present invention canlend itself to wide usage not only for as a business use but also as atoy or game as well as educational usage.

EXAMPLES Example 1

The description will be made with reference to FIGS. 4 and 6.

(a) Preparation of a Liquid Crystal Display Device of ExternallyCharging Type

A deposition aluminum layer of an aluminum deposited film ("#125Metallumy TS" manufactured by Toyo Metallizing Co., Ltd) having asubstrate 2 made of polyethylene terephthalate film was used as aconductive layer 3, on which a solution having the following compositionwas applied, dried and cured as a high-electric resistance transparentlayer 4 with a dry film thickness of 3 μm.:

    ______________________________________                                        polyvinyl formal ("DENKAFORMAL #20" made by                                                                 9.0 g                                           DENKI KAGAKU KOGYO KABUSHIKI KAISHA)                                          10% in tetrahydrofuran solution                                               polyisocyanate ("TAKENATE D110N" made by Takeda                                                             0.4 g                                           Chemical Industries,Ltd.)                                                     ______________________________________                                    

A solution having the following composition was applied, dried and curedwith a dry film thickness of 7 μm so as to form a liquidcrystal-dispersed polymer layer 7 in which liquid crystal droplets arefinely dispersed in polymer matrix.

    ______________________________________                                        polyvinyl formal ("VINYLEC K-624" made by CHISSO                                                            4.2 g                                           CORPORATION) 10% in tetrahydrofuran solution                                  polyisocyanate ("TAKENATE D110N" made by Takeda                                                            0.56 g                                           Chemical Industries,Ltd.)                                                     nematic liquid crystal ("E44" made by MERCK Co.,)                                                          0.36 g                                           ______________________________________                                    

Laminated on the above liquid crystal-dispersed polymer layer was atransparent insulator layer 8 with 9 μm in thickness of polyethyleneterephthalate film ("TETRON film F" made by TEIJIN LIMITED) using anadhesive to complete a liquid crystal display device of externallycharging type 1.

The transparent high-electric resistance layer 4 and the liquidcrystal-dispersed polymer layer 7 which had been adjusted in a 20° C.air at 90% RH were measured on the volume resistivity using a D.C. powersupply ("PR-630" made by TORIO Co.,), an electrometer ("TR8651" made byTakeda Riken Co.,Ltd), an insulating resistance measuring specimenchamber ("TR42" made by ADVANTEST CORPORATION). The result is shownbelow.

    ______________________________________                                        transparent high-electric resistance layer                                                           1.5 × 10.sup.14 Ω · cm            liquid crystal-dispersed polymer layer                                                               7.4 × 10.sup.14 Ω · cm            ______________________________________                                    

(b) Preparation of a Hand-Writable Liquid Crystal Board Set

A hand-writable liquid crystal board set of cord type was prepared asthe following procedures by using the above liquid crystal displaydevice of externally charging type as a liquid crystal board 1.

A conductive layer 3 of a liquid crystal board 1 of externally chargingtype was connected to one terminal 10 of a direct current power source 9which generates a voltage of 600 V between terminals using unillustrated1.5 V drycell (U-3 type), through a series of a resistance 11 of 10 MΩ,a lead wire 12 and another resistance 13 of 10 MΩ. Another terminal 14of the power source 9 was connected to a writing member 20 made of aconductive sponge ("MOSPAC P", a product of FUJIMORI KOGYO CO., LTD.)having a volume resistivity of 5×10⁴ Ω•cm at 50% RH in a 23° C. air,through a series of a lead wire 15, a-contact 16 of a switch forwriting, a lead wire 17, b-contact 18 of a switch for erasing and a leadwire 19. Another conductive sponge ("MOSPAC GS", a product of FUJIMORIKOGYO CO., LTD.) having a volume resistivity of 4×10³ Ω•cm at 50% RH ina 23° C. air was connected as an erasing member 27 through a series oflead wire 21, a b-contact 22 of the switch for writing, a lead wire 23,a-contact 24 of the switch for erasing, resistance 25 of 10 MΩ and alead wire 20 to the lead wire 12.

A timer circuit (not shown) of about a five-minute shut-off time isincorporated as an automatic power-off device between the U-3 type 1.5 vdrycell and the power source 9.

The switch 28 for writing comprises the a-contact 16 and the b-contact18 which link with each other to form an interlock mechanism. The switch31 for erasing comprises the b-contact 22 and the a-contact 24 whichlink with each other to form an interlock mechanism. In a usual state,the b-contact 18 of the switch for erasing and the b-contact 22 of theswitch for writing are in the connected state, whereas the a-contact 16of the switch for writing and the a-contact 24 of the switch for erasingare in the disconnected state.

As described above, the hand-writable liquid crystal board set of thisembodiment was prepared.

(c) Writing Member and Recording Image

Pen type writing member:

A figure was written on a board screen 29 of the liquid crystal board 1using the writing member 20 while the switch 28 for writing beingpressed. As a result, a surface potential arouse in the written portionon the board screen 29 and the thus formed static field made the liquidcrystal of the liquid crystal-dispersed polymer layer 7 align, so that arecording image 30 of the figure could be displayed.

Stamp type writing member:

A stamp 20' was prepared as the writing member 20 using a conductivesponge ("MOSPAC P", a product of FUJIMORI KOGYO CO.,LTD.) having avolume resistivity of 5×10⁴ Ω•cm at 50% RH in a 23° C. air. Use of thestamp 20' made it possible to record and display an image of charactersand figures on the board screen 29 easily at a time.

Pattern film type writing member:

As the writing member 20, a 50 μm polyethylene terephthalate ("TETRONfilm H7W" manufactured by TEIJIN LIMITED) was formed into a pattern film(not shown) having characters, figures etc. written or drawn on itssurface with a transparent conductive ink ("I-201", a productmanufactured by Sumitomo Cement Co., Ltd.). Use of the pattern film madeit possible to record and display an image of characters and patterns onthe board screen 29 easily at a time.

Calligraphic brush type writing member:

As the writing member 20, the aforementioned conductive sponge ("MOSPACP", a product of FUJIMORI KOGYO CO.,LTD ) was formed into a cone-shapeto prepare a calligraphic brush type writing implement. Use of theimplement made it possible to make the trace of the recording image varynarrower or wider as if it were written with a calligraphic brush.

(d) Erasing of Recorded Image

In erasing, a recorded image 30 on the board screen 29 was traced withthe aforementioned erasing member 27 while the switch for erasing beingpressed. As a result, the static charges for the recorded image 30 couldbe removed, facilitating to erase a desired portion alone.

(e) Safety

In the board set of this embodiment, pressing the switch 28 for writingbrings the a-contact 16 of the switch for writing into the connectedstate, which electrically connects the terminal 14 with the writingmember 20. As a result, a voltage arises between the conductive layer 3and the writing member 20, enabling the writing and/or drawing operationonto the liquid crystal board 1. At the same time, the b-contact 22 ofthe switch for writing and the a-contact 24 of the switch for erasingare respectively in the disconnected state, so that the erasing member27 is electrically disconnected from the terminal 10. Therefore, novoltage can arise between the writing member 20 and the erasing member27 while the switch 28 for writing being pressed. Accordingly, noelectric shock was felt by an operator when the operator touched thewriting member 20 and the erasing member 27 by the hands at the sametime while the switch 28 for writing being pressed.

While the switch 31 for erasing is pressed, the a-contact 24 of theswitch for erasing is brought into the connected state, whichelectrically connects the conductive layer 3 with the erasing member 27,and thus these two portions become equipotential. As a result, thestatic charges in the recorded image portion can be removed by rubbingthe board screen 29 with the erasing member 27, enabling the erasure ofthe recorded image 30. Meanwhile, the a-contact 16 of the switch forwriting and the b-contact 18 of the switch for erasing are respectivelyin the disconnected state, so that the writing member 20 is electricallydisconnected from the terminal 14. Therefore, no voltage can arisebetween the writing member 20 and the erasing member 27 while the switch31 for erasing being pressed. Accordingly no electric shock was felt byan operator when the operator touched the writing member 20 and theerasing member 27 by the hands at the same time while the switch 31 forerasing being pressed.

When both the switch 28 for writing and the switch for erasing arepressed at the same time, both the b-contact 18 of the switch forerasing and the b-contact 22 of the switch for writing is brought intothe disconnected state. Accordingly, electrical disconnection is keptbetween the terminal 14 and the writing member 20, and between theterminal 10 and the erasing member 27, respectively.

Therefore, no voltage can arise between the writing member 20 and theerasing member 27 while both the switches being pressed simultaneously.Accordingly, no electric shock was felt by an operator when the operatortouched the writing member 20 and the erasing member 27 by the hands atthe same time while both the switch 28 for writing and the switch 31 forerasing being pressed at the same time.

Still, no shock was felt by an operator when both the charged boardscreen 29 and the erasing member 27 were touched by the hands of theoperator at the same time. This is because the existence of resistances25 and 13 between the erasing member 27 and the conductive layer 3inhibits the current between the erasing member 27 and the board screen29 at most 30 μA.

Moreover, consideration was made for a case in which the conductivelayer 3 of the liquid crystal board was exposed outside. In this case,no shock was felt by the operator when both the writing member 20 andthe exposed portion of the conductive layer 3 were touched by the handsof the operator at the same time. This is because the existence ofresistances 13 and 11 between the conductive layer 3 and the powersource 9 inhibits the current between the writing member 20 and theconductive layer 3 at most 30 μA.

(f) Power saving

In the board set of this example, an automatic power off device isprovided. Therefore, if the hand-writable liquid crystal board set isleft with the power switch on, the power can be automatically cut offafter a predetermined period of time, thus avoiding waste of electricenergy.

Thus, a hand-writable liquid crystal board set of power saving type hasbeen completed in which display and erasing operations can be repeatedlycarried out from the outside of the board screen in a simple manner, andwhich assures such high safety that no electric shock would harm theoperator under any circumstance of the use.

Example 2

Description will be made with reference to FIGS. 5 and 7.

(a) Preparation of a Liquid Crystal Display Device of ExternallyCharging Type

A liquid crystal display device of externally charging type 1 wasprepared in the same manner described in example 1.

The following volume resistivities of layers were obtained in thesimilar measurement as in example 1.

    ______________________________________                                        transparent high-electric resistance layer                                                           1.5 × 10.sup.14 Ω · cm            liquid crystal-dispersed polymer layer                                                               7.4 × 10.sup.14 Ω · cm            ______________________________________                                    

(b) Preparation of a Hand-Writable Liquid Crystal Board Set of CordlessType

A hand-writable liquid crystal board set of cordless type was preparedas the following procedures by using the above liquid crystal displaydevice of externally charging type as a liquid crystal board 1.

A conductive layer 3 of a liquid crystal board 1 of externally chargingtype was connected to one terminal of a direct current power source 9which generates a voltage of 600 V between terminals using unillustrated1.5 V type drycell, through a series of a resistance 11 of 10 MΩ, a leadwire 12 and another resistance 13 of 10 MΩ. Another terminal 14 of thepower source 9 was connected with a series of a lead wire 15, a-contact16 of a switch for writing, a lead wire 17, b-contact 18 of a switch forerasing, and a lead wire 19 to a contact type surface electrode 32 ofthe switch for writing. A contact type surface electrode 33 of a switchfor erasing was connected through a series of a lead wire 21, b-contact22 of the switch for writing, a lead wire 23, a-contact 24 of the switchfor erasing, a resistance 25 of 10 MΩ and a lead wire 26 to the leadwire 12. A writing member 20 and an erasing member 27 were separatelyprovided. The writing member 20 was made of a conductive sponge ("MOSPACP", a product of FUJIMORI KOGYO CO., LTD.) having a volume resistivityof 5×10⁴ Ω•cm at 50% RH in 23° C. air, whereas the erasing member 27 wasmade of a conductive sponge ("MOSPAC GS", a product of FUJIMORI KOGYOCO., LTD.) having a volume resistivity of 4×10³ Ω•cm at 50% RH in 23° C.air.

A timer circuit (not shown) of about a five-minute shut-off time isincorporated as an automatic power-off device between the U-3 type 1.5 vdrycell and the power source 9.

The switch 28 for writing comprises the a-contact 16 and the b-contact22 which link with each other to form an interlock mechanism. The switch31 for erasing comprises the b-contact 18 and the a-contact 24 whichlink with each other to form an interlock mechanism. In an usual state,the b-contact 18 of the switch for erasing and the b-contact 22 of theswitch for writing are in the connected state, respectively, whereas thea-contact 16 of the switch for writing and the a-contact 24 of theswitch for erasing are in the disconnected state, respectively.

As described heretofore, the hand-writable liquid crystal board set ofthis embodiment was prepared.

(c) Writing operation

Pressing the switch 28 for writing by the left hand (with touching thecontact type surface electrode 32 of the switch for writing), anoperator drew lines on a board screen 29 with the writing member 20 heldin the right hand. By this, a recorded image 30 was obtained.

(d) Erasing operation

Pressing the switch 31 for erasing by the left hand (with touching thecontact type surface electrode 33 of the switch for erasing), theoperator traced the recorded image 30 with the erasing member 27 held inthe right hand and erased the image.

In writing and erasing, the operator's body naturally plays a role toelectrically connect the contact type surface electrode 32 of the switchfor writing with the writing member 20, and the contact type surfaceelectrode 33 of the switch for erasing with the erasing member 27,respectively.

(e) Safety

In the board set of this embodiment, pressing the switch 28 for writingbrings the a-contact 16 of the switch for writing into the connectedstate, which electrically connects the terminal 14 with the contact typesurface electrode 32 of the switch for writing. As a result, a voltagearises between the conductive layer 3 and the contact type surfaceelectrode 32. At this time, the contact type surface electrode 32 andthe writing member 20 held in the hand are connected through the humanbody, so that the contact type surface electrode 32 is equipotentialwith the writing member 20. As a result, in a usual writing operation, avoltage arises between the conductive layer 3 and the writing member 20,enabling the writing and/or drawing operation onto the liquid crystalboard 1. Meanwhile the b-contact 22 of the switch for writing and thea-contact 24 of the switch for erasing are respectively in thedisconnected state, so that the contact type surface electrode 33 of theswitch for erasing is electrically disconnected from the terminal 10.Therefore, no voltage can arise between the contact type surfaceelectrode 32 of the switch for writing and the contact type surfaceelectrode 33 of the switch for erasing while the switch 28 for writingbeing pressed. Accordingly, no electric shock was felt by an operatorwhen the operator touched the contact type surface electrode 33 of theswitch for erasing while pressing the switch 28 for writing.

With the switch 31 for erasing being pressed, the a-contact 24 of theswitch for erasing is brought into the connected state, whichelectrically connects the conductive layer 3 with the contact typesurface electrode 33 of the switch for erasing. At this time, thecontact type surface electrode 33 and the erasing member 27 held in thehand are connected through the human body, so that the contact typesurface electrode 33 is equipotential with the erasing member 27. As aresult, in a usual erasing operation, the conductive layer 3 and theerasing member 27 is equipotential. Therefore, the static charges in therecorded image portion can be removed by tracing the board screen 29with the erasing member 27, enabling the erasing of the recorded image30. Meanwhile, the a-contact 16 of the switch for writing and theb-contact 18 of the switch for erasing are respectively in thedisconnected state, so that the contact type surface electrode 32 of theswitch for writing is electrically disconnected from the terminal 14.Therefore, no voltage can arise between the contact type surfaceelectrode 33 of the switch for erasing and the contact type surfaceelectrode 32 of the switch for writing while the switch 31 for erasingbeing pressed. Accordingly, no electric shock was felt by an operatorwhen the operator touched the contact type surface electrode 32 of theswitch for writing simultaneously with pressing the switch 31 forerasing.

When both the switch 28 for writing and the switch 31 for erasing arepressed at the same time, both the b-contact 18 of the switch forerasing and the b-contact 22 of the switch for writing is brought intothe disconnected state. Accordingly, electrical disconnections are keptbetween the terminal 14 and the contact type surface electrode 32 of theswitch for writing, and between the terminal 10 and the contact typesurface electrode 33 of the switch for erasing, respectively.

Therefore, no electric shock was felt by the operator when the operatorpress both the switch 28 for writing and the switch 31 for erasingsimultaneously since no voltage would arise between the contact typesurface electrode 32 of the switch for writing and the contact typesurface electrode 33 of the switch for erasing.

Still, no shock was felt by the operator when both the charged boardscreen 29 and the contact type surface electrode 33 of the switch forerasing were touched by the hands of the operator at the same time. Thisis because the existence of resistances 25 and 13 between the contacttype surface electrode 33 of the switch for erasing and the conductivelayer 3 inhibits the current between the contact type surface electrode33 of the switch for erasing and the board screen 29 not more than 30μA.

Moreover, consideration was made for a case in which the conductivelayer 3 of the liquid crystal board 1 was exposed outside. In this case,no shock was felt by the operator when both the contact type surfaceelectrode 32 of the switch for writing and the exposed portion of theconductive layer 3 were touched by the hands of the operator at the sametime. This is because the existence of resistances 13 and 11 between theconductive layer 3 and the power source 9 inhibits the current betweenthe contact type surface electrode 32 of the switch for writing and theconductive layer 3 not more than 30 μA.

(f) Power saving

In the board set of this example, an automatic power off device isprovided. Therefore, if the hand-writable liquid crystal board set isleft with the power switch on, the power can be automatically cut offafter a predetermined period of time, thus avoiding waste of electricenergy.

Thus, a hand-writable liquid crystal board set of power saving type hasbeen completed in which displaying and erasing operations can berepeatedly carried out from the outside of the board screen in a simplemanner, which assures such high safety that no electric shock would harmthe operator under any circumstance of the use. In addition the set hasa high durability free from troubles such as entanglement ordisconnections of cords since no cord is employed.

Example 3

(a) Preparation of a Liquid Crystal Display Device of ExternallyCharging Type

A liquid crystal display device of externally charging type 1 wasprepared in the similar manner with example 1 except that a polyethyleneterephthalate film of 2 μm thick ("KX43-4 2.0W", a product of DiafoilCo.,Ltd.) were laminated as a transparent high-electric resistance layer4 on the same conductive layer 3 in example 1.

The following volume resistivity of the layer was obtained in thesimilar measurement as in example 1.

    ______________________________________                                        transparent high-electric resistance layer                                                           7.5 × 10.sup.14 Ω · cm            ______________________________________                                    

(b) Preparation of a Hand-Writable Liquid Crystal Board Set

A hand-writable liquid crystal board set was prepared as the same mannerin example 1 by using the above liquid crystal display device ofexternally charging type as a liquid crystal board 1.

Example 4

(a) Preparation of a Liquid Crystal Display Device of ExternallyCharging Type

A liquid crystal display device of externally charging type 1 wasprepared in the similar manner with example 1. A solution having thefollowing composition was applied on the surface of the transparentinsulator layer 8 of the device as a dirt-protecting film (not shown) soas to be 3 μm in thick after dried. Then the solution was dried andcured.

    ______________________________________                                        polyol ("CEFRAL COAT A-201TB" made by                                                                      9.0 g                                            CENTRAL GLASS CO., LTD.)                                                      hexamethylene diisocyanate isocyanurate (polyisocyanate:                                                   1.0 g                                            made by CENTRAL GLASS CO., LTD.)                                              ethyl acetate                16.0 g                                           ______________________________________                                    

(b) Preparation of a Hand-Writable Liquid Crystal Board Set

A hand-writable liquid crystal board set was prepared as the same mannerin example 1 by using the above liquid crystal display device ofexternally charging type as a liquid crystal board 1.

Comparative Example 1

(a) Preparation of a Liquid Crystal Display Device of ExternallyCharging Type

A liquid crystal display device of externally charging type 1 wasprepared in the similar manner with example 1 except that no transparenthigh-electric resistance layer 4 was provided on conductive layer 3 ofexample 1 .

(b) Preparation of a Hand-Writable Liquid Crystal Board Set

A hand-writable liquid crystal board set was prepared as the same mannerin example 1 by using the above liquid crystal display device ofexternally charging type as a liquid crystal board 1.

Comparative Example 2

(a) Preparation of a Liquid Crystal Display Device of ExternallyCharging Type

Applied on the same conductive layer 3 with that of example 1 was amixture of compositions listed below to form a liquid crystal-dispersedpolymer layer. A polyethylene terephthalate film of 9 μm thick ("TETRONfilm F", a product of TEIJIN LIMITED) was pressed as a transparentinsulator layer 8 over the above applied film. The film side of thedevice was exposed by a UV-irradiating apparatus for one minute suchthat light-curing occurred whereby a liquid crystal-dispersed polymerlayer 7 was formed. Thus a liquid crystal display device of externallycharging type was prepared.

    ______________________________________                                        epoxyacrylate ("ARONIX M210", a product by                                                                1.2 g                                             Toagosei Chemical Industry Co.,Ltd.)                                          2-ethylhexylacrylate        1.8 g                                             benzoin-isopropylether      0.06 g                                            nematic liquid crystal ("E7" made by MERCK Co.)                                                           7.0 g                                             sphere particles of 12 μm in diameter ("MBX-12S"                                                       0.005 g                                           made by SEKISUI PLASTICS Co.,Ltd.)                                            ______________________________________                                    

The volume resistivity of the layer below was obtained in the similarmeasurement as in example 1.

    ______________________________________                                        liquid crystal-dispersed polymer layer                                                              5.6 × 10.sup.9 Ω · cm              ______________________________________                                    

(b) Preparation of a Hand-Writable Liquid Crystal Board Set

A hand-writable liquid crystal board set was prepared as the same mannerin example 1 by using the above liquid crystal display device ofexternally charging type as a liquid crystal board 1.

Comparative Example 3

(a) Preparation of a Liquid Crystal Display Device of ExternallyCharging Type

A liquid crystal display device of externally charging type 1 wasprepared in the similar manner as in example 1 , except in that anaqueous solution of polyvinyl alcohol ("Gohsenol GH-17", a product byThe Nippon Synthetic Chemical Industry Co.,Ltd.) was applied and thendried in place of the transparent high-electric resistance layer 2 so asto be 2 μm in thick after dried.

The volume resistivity of the polyvinyl alcohol was 2.1×10¹¹ Ω•cm at 90%RH in a 20° C. air.

(b) Preparation of a Hand-Writable Liquid Crystal Board Set

A hand-writable liquid crystal board set was prepared as the same mannerin example 1 by using the above liquid crystal display device ofexternally charging type as a liquid crystal board 1.

Comparative Example 4

(a) Preparation of a Liquid Crystal Display Device of ExternallyCharging Type

A liquid crystal display device of externally charging type 1 wasprepared in the similar manner with example 1 except that no transparentinsulator layer 8 was provided.

(b) Preparation of a Hand-Writable Liquid Crystal Board Set

A hand-writable liquid crystal board set was prepared as the same mannerin example 1 by using the above liquid crystal display device ofexternally charging type as a liquid crystal board 1.

The following items were evaluated for each hand-writable liquid crystalboard set of the above examples and comparative examples.

(1) Recording performance:

Each hand-writable liquid crystal board set was left in an environmenthaving a relative humidity (to be abbreviated as RH) of 90% in a 20° C.air, to absorb the moisture. With each of the thus damped sets, anoperator draw a recording image 30 on the board screen 29 with thewriting member 20 while the switch 28 for writing was pressed. Then thedifference of the density between the image area and the image-free areawas measured using a Macbeth density-meter (RD-915 type)

Good: The difference of O.D value between the image area and theimage-free area is 0.4 or more.

Bad: The difference of O.D value between the image area and theimage-free area is less than 0.4.

(2) Image retention memory performance

Each of the hand-writable liquid crystal board set with the image 30 wasleft at 90% RH in a 20° C. air, and the image retention time of therecorded image 30 was measured.

Good: The image is retained for 30 minutes or longer with keeping thedifference of O.D value between the image area and the image-free areaat or over 0.4.

Medium: The image is retained for at least 10 minutes and less than 30minutes with keeping the difference of O.D value between the image areaand the image-free area at or over 0.4.

Bad: The image is retained for less than 10 minutes with keeping thedifference of O.D value between the image area and the image-free areaat or over 0.4.

(3) Erasability

Each hand-writable liquid crystal board set was left in a 20° C. air at90% RH, to absorb the moisture. With each of the thus damped sets, anoperator draws a recording image 30 on the board screen 29 with thewriting member 20 while the switch 28 for writing being pressed. Thenthe recorded image 30 was erased by tracing it with the erasing member27 while the switch 31 for erasing being pressed.

Good: The image area erased can hardly be seen (The O.D value of theerased area is less than the O.D value of the image-free area plus0.05.)

Bad: The image area eraser is clearly known (The O.D value of the erasedarea is the 0.D value of the image-free area plus 0.05 or more.)

(4) Strength of the board

Each liquid crystal board was rubbed on its surface with a materialhaving a pencil strength of 2H, and the damage was observed.

Good: No damage

Bad: The liquid crystal-dispersed layer is peeled off and damaged.

(5) Dirt-protecting performance

Each of the hand-writable liquid crystal board sets had been left in aroom environment for about one month while the writing and erasingoperations were repeated. Thereafter, the above items (2) Imageretention memory performance and (3) Erasability was reevaluated.

Good: No deterioration in the image retention memory performance anderasability.

Bad: deterioration in the image retention memory performance anderasability is observed.

The result of the evaluation is shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                    Image                    Dirt-                                    Recording   memory            Strength                                                                             protect-                                 per-        per-     Eras-    of     ing per-                                 formance    formance ability  board  formance                                 ______________________________________                                        Ex.1  Good      Good     Good   Good   Bad                                    Ex.2  Good      Good     Good   Good   Bad                                    Ex.3  Good      Good     Good   Good   Bad                                    Ex.4  Good      Good     Good   Good   Good                                   CEx.1 Good      Medium   Bad    Good   Bad                                    CEx.2 Good      Bad      Bad    Good   Bad                                    CEx.3 Good      Medium   Bad    Good   Bad                                    CEx.4 Good      Good     Bad    Bad    Bad                                    ______________________________________                                         Ex: Example; CEx: Comparative example                                    

What we claim is:
 1. A liquid crystal display device of externallycharging type comprising:a conductive layer; a transparent high-electricresistance layer of a crosslinking polymer having a volume resistivityof 10¹³ Ω•cm or more in 20° C. air at a relative humidity of 90%; aliquid crystal-dispersed polymer layer in which liquid crystal is finelydispersed in a polymer matrix, said layer having a volume resistivity of10¹³ Ω•cm or more in 20° C. air at a relative humidity of 90%; and atransparent insulator layer having a volume resistivity of 10¹³ Ω•cm ormore in 20° C. air at a relative humidity of 90% wherein all of saidlayers are successively layered in the order presented.
 2. The liquidcrystal display device of externally charging type according to claim 1,wherein the polymer matrix of said liquid crystal-dispersed polymerlayer is composed of a polymer having a cross-linking structure.
 3. Theliquid crystal display device according to claim 1, wherein saidcross-linking polymer making up the high-electric resistance layer isobtained by reacting a polymer containing at least one functional group,wherein said functional group is selected from the group consisting of adouble bond, a nitrile group, a mercapto group, a hydroxyl group, acarboxyl group, an epoxy group, an isocyanate group, an amino group anda chlorosulfone group, with a cross-linking agent reactive with saidfunctional group.
 4. The liquid crystal display device according toclaim 3, wherein said polymer is a silicone polymer.
 5. The liquidcrystal display device according to claim 1, wherein said cross-linkingpolymer making up the high-electric resistance layer is obtained byreacting a polymer containing at least one functional group, whereinsaid functional group is selected from the group consisting of a doublebond, a nitrile group, a mercapto group, a hydroxyl group, a carboxylgroup, an epoxy group, an isocyanate group, an amino group and achlorosulfone group, with a reactive polymer to form a cross-linkingpolymer.
 6. The liquid crystal display device according to claim 5,wherein said polymer is a silicone polymer.
 7. The liquid crystaldisplay device according to claim 5, wherein said reactive polymercomprises at least one polymer selected from the group consisting ofphenol resin, amino resin, polyisocyanate, polyol and epoxy resin. 8.The liquid crystal display device according to claim 1, wherein saidcross-linking polymer making up the high-electric resistance layer isformed by reacting one of a diisocyanate and a polyisocyanate with atleast one polymer selected from the group consisting of apolyvinylacetal resin; an epoxy resin; acrylic resins having a hydroxygroup or a carboxyl group; a polyester resin; and a fluororesin.
 9. Theliquid crystal display device according to claim 1, wherein saidcross-linking polymer making up the high-electric resistance layer isformed by reacting one of a diisocyanate and a polyisocyanate withpolyvinylacetal.
 10. The liquid crystal display device according toclaim 1, wherein the polymer matrix of said liquid crystal-dispersedpolymer layer is composed of a cross-linking polymer which is obtainedby reacting a polymer containing at least one functional group, whereinsaid functional group is selected from the group consisting of a doublebond, a nitrile group, a mercapto group, a hydroxyl group, a carboxylgroup, an epoxy group, an isocyanate group, an amino group and achlorosulfone group, with a cross-linking agent reactive with saidfunctional group.
 11. The liquid crystal display device according toclaim 10, wherein said polymer is a silicone polymer.
 12. The liquidcrystal display device according to claim 1, wherein said polymer matrixof said liquid crystal-dispersed polymer layer comprises a cross-linkingpolymer which is obtained by reacting a polymer containing at least onefunctional group, wherein said functional group is selected from thegroup consisting of a double bond, a nitrile group, a mercapto group, ahydroxyl group, a carboxyl group, an epoxy group, an isocyanate group,an amino group and a chlorosulfone group, with a reactive polymer toform a cross-linking polymer.
 13. The liquid crystal display deviceaccording to claim 12, wherein said polymer is a silicone polymer. 14.The liquid crystal display device according to claim 12, wherein saidreactive polymer is selected from the group consisting of phenol resin,amino resin, polyisocyanate, polyol and epoxy resin.
 15. The liquidcrystal display device according to claim 1, wherein said polymer matrixof said liquid crystal-dispersed polymer layer comprises a cross-linkingpolymer formed by reacting one of a diisocyanate and a polyisocyanatewith at least one polymer selected from the group consisting of apolyvinylacetal resin; an epoxy resin; acrylic resins having a hydroxygroup or a carboxyl group; a polyester resin; and a fluororesin.
 16. Theliquid crystal display device of externally charging type according toclaim 1, wherein said polymer matrix of said liquid crystal-dispersedpolymer layer comprises a cross-linking polymer formed by reacting oneof a diisocyanate and a polyisocyanate with polyvinylacetal.
 17. Ahand-writable liquid crystal board set comprising:(A) a liquid crystalboard of the externally charging type, said liquid crystal boardcomprising: a conductive layer; a transparent high-electric resistancelayer of a cross-linking polymer having a volume resistivity of 10¹³Ω•cm or more in 20° C. air at a relative humidity of 90%; a liquidcrystal-dispersed polymer layer in which liquid crystal is finelydispersed in a polymer matrix, said liquid crystal-dispersed polymerlayer having a volume resistivity of 10¹³ Ω•cm or more in 20° C. air ata relative humidity of 90%; and a transparent insulator layer having avolume resistivity of 10¹³ Ω•cm or more in 20° C. air at a relativehumidity of 90%, wherein all of said layers are successively layered inthe order presented; (B) display means for displaying at least onerecorded image onto a surface of said liquid crystal board; (C) erasingmeans for erasing said recorded image, said erasing means having anerasing member electrically connected to said conductive layer; (D) apower source for generating a surface potential between the surface ofsaid liquid crystal board and said conductive layer; and (E) a writingmember electrically connected to one terminal of said power source, saidpower source having another terminal connected to said conductive layer.18. The hand-writable liquid crystal board set according to claim 17,wherein the polymer matrix of said liquid crystal-dispersed polymerlayer is composed of a polymer having a cross-linking structure.
 19. Thehand-writable liquid crystal board set according to claim 17, whereinsaid writing member, said erasing member, and the electrical connectionsthereto all have a respective volume resistivity of 10¹² Ω•cm or less ina 23° C. at a relative humidity of 50%.
 20. The hand-writable liquidcrystal board set according to claim 17, wherein an electric resistanceof 200 KΩ to 100 MΩ is electrically connected at least between oneterminal of said power source and said conductive layer, between oneterminal of said power source and said erasing member, and between saidconductive layer and said erasing member.
 21. The hand-writable liquidcrystal board set according to claim 17, wherein an electric circuitincluding said display means and said erasing means is equipped with aninterlock device for preventing the occurrence of electric shocks. 22.The hand-writable liquid crystal board set according to claim 17,wherein an electric circuit including said display means and saiderasing means is equipped with an automatic power-off device.
 23. Thehand-writable liquid crystal board set according to claim 17, whereinthe surface of said board has been subjected to a dirt-protectiontreatment.
 24. The hand-writable liquid crystal board set according toclaim 17, wherein said writing member comprises a stamp having a patternof a conductive material formed thereon.
 25. The hand-writable liquidcrystal board set according to claim 17, wherein said writing member andsaid erasing member have been subjected to a dirt-protection treatment.26. The hand-writable liquid crystal board set according to claim 17,wherein said erasing member comprises a stamp having a pattern of aconductive material formed thereon.
 27. The hand-writable liquid crystalboard set according to claim 17, wherein said cross-linking polymermaking up the high-electric resistance layer is obtained by reacting apolymer containing a functional group, wherein said functional group isselected from the group consisting of a double bond, a nitrile group, amercapto group, a hydroxyl group, a carboxyl group, an epoxy group, anisocyanate group, an amino group and a chlorosulfone group, with across-linking agent reactive with said functional group.
 28. Thehand-writable liquid crystal board set according to claim 27, whereinsaid polymer is a silicone polymer.
 29. The hand-writable liquid crystalboard set according to claim 17, wherein said cross-linking polymermaking up the high-electric resistance layer is obtained by reacting apolymer containing at least one functional group, wherein saidfunctional group is selected from the group consisting of a double bond,a nitrile group, a mercapto group, a hydroxyl group, a carboxyl group,an epoxy group, an isocyanate group, an amino group and a chlorosulfonegroup, with a reactive polymer to form a cross-linking polymer.
 30. Thehand-writable liquid crystal board set according to claim 29, whereinsaid polymer is a silicone polymer.
 31. The hand-writable liquid crystalboard set according to claim 29, wherein said reactive polymer comprisesat least one polymer selected from the group consisting of phenol resin,amino resin, polyisocyanate, polyol and epoxy resin.
 32. Thehand-writable liquid crystal board set according to claim 17, whereinsaid cross-linking polymer making up the high-electric resistance layeris formed by reacting one of a diisocyanate and a polyisocyanate with atleast one polymer selected from the group consisting of apolyvinylacetal resin; an epoxy resin; acrylic resins having a hydroxygroup or a carboxyl group; a polyester resin; and a fluororesin.
 33. Thehand-writable liquid crystal board set according to claim 17, whereinsaid cross-linking polymer making up the high-electric resistance layeris formed by reacting one of a diisocyanate and a polyisocyanate withpolyvinylacetal.
 34. The hand-writable liquid crystal board setaccording to claim 17, wherein the polymer matrix of said liquid crystaldispersed polymer layer is composed of a cross-linking polymer which isobtained by reacting a polymer containing at least one functional group,wherein said functional group is selected from the group consisting of adouble bond, a nitrile group, a mercapto group, a hydroxyl group, acarboxyl group, an epoxy group, an isocyanate group, an amino group anda chlorosulfone group, with a cross-linking agent reactive with saidfunctional group.
 35. The hand-writable liquid crystal board setaccording to claim 34, wherein said polymer is a silicone polymer. 36.The hand-writable liquid crystal board set according to claim 17,wherein said polymer matrix of said liquid crystal-dispersed polymercomprises a cross-linking polymer which is obtained by reacting apolymer containing at least one functional group, wherein saidfunctional group is selected from the group consisting of a double bond,a nitrile group, a mercapto group, a hydroxyl group, a carboxyl group,an epoxy group, an isocyanate group, an amino group and a chlorosulfonegroup, with a reactive polymer to form a cross-linking polymer.
 37. Thehand-writable liquid crystal board set according to claim 36, whereinsaid polymer is a silicone polymer.
 38. The hand-writable liquid crystalboard set according to claim 36, wherein said reactive polymer isselected from the group consisting of phenol resin, amino resin,polyisocyanate, polyol and epoxy resin.
 39. The hand-writable liquidcrystal board set according to claim 17, wherein said polymer matrix ofsaid liquid crystal-dispersed polymer layer comprises a cross-linkingpolymer formed by reacting one of a diisocyanate and a polyisocyanatewith at least one polymer selected from the group consisting of apolyvinylacetal resin; an epoxy resin; acrylic resins having a hydroxygroup or a carboxyl group; a polyester resin; and a fluororesin.
 40. Thehand-writable liquid crystal board set according to claim 17, whereinsaid polymer matrix of said liquid crystal dispersed polymer layercomprises a cross-linking polymer formed by reacting one of adiisocyanate and a polyisocyanate with polyvinylacetal.